In general, an electro-hydraulic excavator expands and contracts a boom cylinder or the like by a hydraulic fluid discharged from an electro-hydraulic actuator (hydraulic pump-motor) operated according to the drive of an electric motor-generator (hereinafter, referred to as “electric motor”) to manipulate a work apparatus. In other words, the expansion and contraction of the boom cylinder can be controlled according to the forward and reverse rotation of the electric motor. A high pressure is generated in a large chamber of the boom cylinder due to its own weight during a boom-down operation in which the boom descends or is lowered. In addition, the hydraulic pump-motor is driven by the hydraulic fluid fed back thereto from the large chamber of the boom cylinder to cause the electric motor to generate electricity.
FIG. 3 is a graph showing the characteristics of the electro-hydraulic actuator (EHA).
In FIG. 3, a graph curve (a) represents the number of rotations and a torque, which can be generated by the electric motor constituting the electro-hydraulic actuator. A torque that can be generated by the electric motor which was applied with power is substantially equal to a torque that can be applied to the outside during the generation of electricity.
In FIG. 3, a graph curve (b) represents necessary energy converted into the number of rotations and a torque of the electric motor constituting the electro-hydraulic actuator in the case where an excavator receives a force from an engine to drive the work apparatus. In other words, since the graph curve (a) includes the entire section of the graph curve (b), a hybrid excavator having the electro-hydraulic actuator mounted thereon can generate a driving speed and a force larger than those of a hydraulic excavator that receives a torque from the engine to drive the work apparatus.
Meanwhile, in a hybrid excavator, in the case where the work apparatus is decelerated or descends by its own weight such as the boom-down operation, it is not operated by controlling a meter-out valve, but by receiving energy from the engine. For this reason, even when the work apparatus is operated at high speed, it can be stopped at high torque. If the energy is converted into the number of rotations and the torque of the electric motor-generator, it includes a hatched region (d).
In other words, during a high speed boom-down operation, the energy is equal to a speed and a torque corresponding to a point (c) where the graph curves (a) and (b) intersect with each other, and thus the work apparatus can be brought to a quick stop by closing the valve in the hydraulic excavator.
On the other hand, in the hybrid excavator having the electro-hydraulic actuator mounted thereon, when the work apparatus (i.e., a boom) performs a high speed boom-down operation (in this case, a point (e) requires a pressure higher than that in the point (c) where the graph curves (a) and (b) intersect with each other), the torque of the electric motor-generator being operated cannot be increased any longer, which makes it impossible to bring the work apparatus to a quick stop.
For this reason, in the case where a work such as pipe laying is performed with the aid of a worker, the work apparatus is not brought to a quick stop upon occurrence of a dangerous situation, thereby causing a safety accident.